The most common means of artificial lift of fluid from a wellbore such as, but limited to, oil and gas wells, is a sucker rod supported downhole reciprocating pump driven by a beam pumping unit, such as illustrated in FIG. 1. A conventional beam pumping unit uses an arrangement of high-torque gears, cranks, levers, and linkages to convert the rotary motion of a drive motor into the reciprocating motion needed to operate the downhole pump. These moving components, especially the high-torque gears, contribute significantly to the capital cost of this type of unit. This arrangement of gears, cranks, and levers also produces a load (torque) of the drive motor that varies widely throughout the pumping cycle. This reduces pumping efficiency because the drive motor operates at less than full-load during most of the pumping cycle.
Many variations and alternatives to beam pumping units have been developed in an attempt to reduce equipment capital costs and at the same time increase pumping efficiency. One such effort was directed to hydraulic powered pumping units wherein the reciprocating motion needed to operate the downhole pump is obtained from an upstanding hydraulic power cylinder which contains a piston whose shaft is connected to a sucker rod string. Samples of such equipment are fully and completely disclosed in "Primer of Oil and Gas Production", published by the American Petroleum Institute, Dallas, Tex., 1962, pages 24 and 25, and "What's New in Artificial Lift", World Oil, May 1989, pages 30 and 31. Current commercially available hydraulic pumping units employ a power cylinder as aforesaid coupled with a compressed gas assist (booster) for the power cylinder as will be disclosed in greater detail hereinafter with reference to FIGS. 2 and 3. The use of hydraulics in lieu of gears, cranks, and the like gives these pumping units a lower capital cost than beam pumping units. For example, compressed nitrogen boosted hydraulic pumping units are available at about 70% of the cost of an equivalent mechanical unit. The elimination of gears, cranks, and the like also allows the cyclic variations in drive motor torque to be minimized, thereby allowing the drive motor to run more efficiently. For example, nitrogen boosted hydraulic pumping units may allow up to a 30% reduction in operating costs.
In general, current operation employing compressed gas-boosted hydraulic pumping units employ compressed nitrogen, for example, so that the pressure of the nitrogen is such that the force exerted by the gas is insufficient to lift the load on the upstroke, but more than sufficient to meet the load or energy requirement for the downstroke. Thus, the hydraulic pump used to actuate the piston in the power cylinder is required to work on the upstroke to lift the load and work on the downstroke to overcome the compressed nitrogen spring and further compress the nitrogen for the next upstroke. The pressure of the nitrogen gas in the booster or spring is adjusted so that the work done by the hydraulic pump on the upstroke is approximately the same as the work done by the same pump during the downstroke. This results in a relatively constant torque for the motor driving the hydraulic pump. This in turn allows that drive motor to operate more efficiently.
Two nitrogen boosted hydraulic pumping units that are currently available commercially will be described in greater detail hereinafter in FIGS. 2 and 3. However, it is important to note that both units use fixed displacement hydraulic pumps in association with solenoid-operated valves to control pumping direction. They also use nitrogen boosters in parallel with the hydraulic pump to assist in pumping as will be disclosed hereinafter in greater detail.
Fixed displacement hydraulic pumps are a species of the genus referred to as positive displacement hydraulic pumps, see "Using Industrial Hydraulics", published by Hydraulics and Pneumatics Magazine, Cleveland, Ohio, 2nd Edition, 1984, page 6-5. A fixed displacement hydraulic pump within the positive displacement hydraulic pump genus is a positive displacement design in which the amount of displacement cannot be varied. At a given input RPM this type of pump must deliver hydraulic fluid flow in an amount equivalent to its fixed displacement. A separate species of pump within the positive displacement hydraulic pump genus is the variable displacement pump which is a positive displacement design in which the amount of hydraulic fluid displacement from the pump can be easily changed.
The amount of hydraulic fluid output flow that is delivered by a fixed displacement pump can be changed only by changing the drive speed of the drive motor for that pump. A positive displacement hydraulic pump whose output flow of hydraulic fluid (otherwise referred to as "displacement") can be varied has numerous advantages in the context of downhole well pumping. The displacement can be varied simply through adjustment by manual means or it can be totally automated and interfaced with computerized programming. Another advantage of the variable displacement hydraulic pump is that heat is not generated by moving hydraulic fluid around a circuit when no work is being done by the hydraulic pump. Even when a fixed displacement hydraulic pump is unloaded, energy is converted into heat simply because the hydraulic fluid is in motion. Likewise, during operation, hydraulic fluid must be diverted, restricted, or removed from the system at a high pressure level. Many times, this pressure level is considerably higher than the actual pressure required to do the work during most of the cycle. On the other hand, the variable displacement pump can be made to produce only the hydraulic energy required to do the work. It can also be made to produce this energy only when it is needed to cause the required motion of the load.
In the context of downhole well pumping, the singular reliance by the prior art upon fixed displacement hydraulic pumps gives the result that the power cylinder drives its piston (and the downhole pump) at a constant velocity so that at the end of the piston stroke when the power cylinder reverses the direction of movement of the piston the reversal is very abrupt. This causes high decelerations and accelerations which induces undesired dynamic loads on the sucker rod string that connects the power cylinder piston to the downhole pump. The results are shorter sucker rod life and an increased number of well workovers.
With a variable displacement hydraulic pump the volume of hydraulic fluid output displaced per revolution of the pump can be varied from zero to full flow while the pump is running. With this capability, near the end of the stroke of the power cylinder piston the velocity of the sucker rod string and downhole pump may be slowed in a smooth and controlled fashion. When the sucker rod string has stopped at the end of the stroke it may then be accelerated in the opposite direction in a similarly smooth and controlled manner. This results in greatly reduced dynamic loading of the sucker rod string and other associated equipment which prolongs the life of the pumping unit, sucker rod string, and downhole pump, and reduces the number of well workovers required.
Also, the prior art employs compressed gas boosters so that they operate in parallel with the fixed displacement hydraulic pumps which requires an extra cylinder which is an expensive item of equipment.